When a color image is formed on a transparent sheet with a white color material as background color material and C, M, Y, K color materials as process color materials and has colors determined by the total amount of the color materials that are applied per unit area, image formation failures such as ink drooping and ink fixation failures are prevented from occurring. Of four colors or more including white represented by a CMYKW 1-bit pixel signal, the three colors C, M, Y (gray) are replaced with the color K. After the replacement, for three or more of C, M, Y, K, no white color ink is applied since the transparent sheet is opaque without the white color ink. This control process is effective to reduce the number of applications, i.e., the total amount of inks applied, thereby preventing image formation failures such as ink drooping from occurring.